Method of making an imaging fibre apparatus and optial fibre apparatus with different core

ABSTRACT

A method of forming an imaging fibre apparatus comprises arranging rods to form a plurality of stacks each comprising a respective plurality of rods, wherein: for each stack, the respective plurality of rods comprises rods having different core sizes, the rods of different core sizes being arranged in a selected arrangement, and the rods of different core sizes being arranged such that each stack has a respective selected shape; wherein the selected shape or shapes are such that the stacks stack together in a desired arrangement; the method further comprising: drawing each of the plurality of stacks; stacking together the plurality of drawn stacks together in the desired arrangement to form a further stack; drawing the further stack; and using the drawn further stack to form an imaging fibre apparatus, wherein the selected arrangement of the rods in each stack and the selected shape or shapes of the stacks are such that the further stack comprises a repeating pattern of rods of different core sizes.

This Application is a Divisional of application Ser. No. 16/479,273filed on Jul. 19, 2019 which is a National Stage of ApplicationPCT/GB2018/050173 filed Jan. 19, 2018 which claims priority toApplication EP1700936.6 filed on Jan. 19, 2017. The entire contents ofthese applications are incorporated herein by reference in theirentirety.

FIELD

The present invention relates to an optical fibre apparatus, for examplea spatially coherent imaging fibre, and a method of making an opticalfibre apparatus.

BACKGROUND

A coherent imaging fibre (which may be referred to as a fibre bundle)may comprise many thousands of light guiding cores, each of whichtransmits a part of an image along the fibre length. Each core may actas a pixel to build up an image.

In order to build up high resolution images, the cores of the fibre maybe placed close together. There may be a limit to how close to eachother the cores may be placed. When cores get too close together, lightin one core may couple out of that core and into another core. Suchcoupling of light between cores may degrade a transmitted image.

One method of reducing core to core coupling may be to increase thenumerical aperture of the cores. However, glass manufacturinglimitations may dictate how cost effective a method comprisingincreasing the numerical aperture may be. Increasing the numericalaperture of the cores may lead to background fluorescence.

Fibres may be formed using germanium doped silica. However, theproduction of fibres comprising germanium doped silica may be difficultand/or involve high stresses.

Another method of reducing core to core coupling may be to make adjacentcores dissimilar. For example, adjacent cores may be made from differentmaterials. Adjacent cores may have different sizes.

An imaging fibre may be created by randomly packing different corestogether in a jacket tube and drawing the cores and jacket tube tofibre. However, random packing may result in some nearest neighbourcores that are the same, due to the random nature of the coredistribution. Core to core coupling may occur between nearest neighbourcores.

Fibres available from Schott AG stack arrays of uniform cores made fromspecialty glasses with high index contrasts compared with the claddingglasses. Schott AG also provide fibres having absorbing interstitialelements or leached fibre bundles where the interstitial glass is etchedaway, leaving a bundle of isolated cores joined at either end of thefibre and separated by air along the fibre length.

Fujikura, Ltd produce imaging fibres based on doped silica glasses inwhich cross talk is suppressed by using high NA (−0.4) step cores with arandom variation in size and random spatial distribution. It may bedifficult to acquire the raw materials to fabricate such a fibreeconomically. This may result in high manufacturing costs.

The widespread use of endoscopic imaging fibres is now commonplace inbiology and medicine. These fibres may allow microscopy, rapid imagingof tissues and surgical guidance, potentially reducing the need forinvasive removal of tissue for diagnosis. Originally developed in thelate 1950s, current clinical state-of-the-art endoscopic imaging fibrescost several thousands of dollars, need to be sterilized between usesand have limited use life cycles. For example, in some current systems,an upper limit to the number of uses may be 20 uses, after which theimaging fibre may need to be replaced at significant cost.

SUMMARY

In a first aspect of the invention, there is provided a method offorming an optical fibre apparatus comprising arranging rods to form aplurality of stacks each comprising a respective plurality of rods. Foreach stack, the respective plurality of rods may comprise rods havingdifferent core sizes, the rods of different core sizes being arranged ina selected arrangement, and the rods of different core sizes beingarranged such that each stack has a respective selected shape. Theselected shape or shapes may be such that the stacks stack together in adesired arrangement. The method may further comprise: drawing each ofthe plurality of stacks; stacking together the plurality of drawn stackstogether in the desired arrangement to form a further stack; and drawingthe further stack. The method may comprise using the drawn further stackto form an optical fibre apparatus. The optical fibre apparatus maycomprise an imaging fibre apparatus.

The selected arrangement of the rods in each stack and the selectedshape or shapes of the stacks may be such that the further stackcomprises a repeating pattern of rods of different core sizes. Therepeating pattern may extend across all, or at least a plurality, of thestacks. Substantially the same repeating pattern may be obtained forall, or at least a plurality, of the stacks. Thus, the optical fibreapparatus may comprises a repeating pattern of rods of different coresizes, for example across substantially all of its fibre cross-sectionalarea.

Including rods having different core sizes in each stack may reducecross-talk between the rods in the stack, when compared with a stack inwhich the rods are of the same size.

Using cores having dissimilar core sizes instead of other methods ofcross-talk reduction may allow lower cost materials to be used. In somecircumstances, a lower cost material with dissimilar cores may havesimilar performance to that of a higher cost material having a uniformcore size.

The rods may comprise any material that may be used to form an opticalfibre, for example any suitable fibre that may be subject to a drawingprocess to produce an optical fibre having desired properties. The stackmay comprise any suitable arrangement of the rods, for example anysuitable arrangement in which the rods are arranged next to each otherand substantially parallel along their lengths. In some arrangementsadditional material may be provided between the rods.

Each plurality of rods may comprise rods having at least three differentcore sizes.

The selected shape or shapes may be such that the stacks, when drawn,stack together in a desired arrangement. The desired arrangement maycomprise a tiling. The desired arrangement may comprise a space-fillingarrangement. The shape or shapes may comprise any combination of shapesthat may be tiled or tessellated in a plane.

Each rod may comprise a core and cladding.

For each stack, the respective plurality of rods may comprises rodshaving different outer sizes. The different outer sizes may comprisedifferent cladding sizes. The different outer sizes may comprise atleast one of: different outer diameters, different outer cross-sections,different cladding diameters, different cladding cross-sections.

For each stack, each of the respective plurality of rods may havesubstantially the same ratio of core size to outer size. Rods ofdifferent core sizes may each have substantially the same ratio of coresize to cladding size. For example, each rod may have the same ratio ofcore diameter to cladding diameter.

The method may further comprise obtaining the rods by drawing at leastone preform.

The obtaining of the rods may comprise drawing a selected type ofpreform. Different core sizes may be obtained by drawing the sameselected type of preform differently. For example, different core sizesmay be obtained by drawing down the same selected type of preform atdifferent speeds. A single preform may be drawn into different sizes ofrod by changing a drawing speed, without making other changes, forexample rejacketing.

By obtaining the different core sizes from a single, selected type offibre preform, cost and/or complexity may be reduced when compared withmethods in which rods are drawn down from different fibre preforms.

The selected type of fibre preform may comprise different fibre preformsmade of substantially the same materials. The single type of fibrepreform may comprise fibre preforms having substantially the same outerdiameter and core diameter. The rods may be formed of different fibrepreforms of substantially the same type, or from a single length ofselected preform.

Using different sizes of rod that are drawn from the same preform mayresult in a coherent imaging fibre that has a desired imagingperformance at low cost. Cross-talk between the imaging cores may bereduced by the difference in size between adjacent rods.

The method may comprise jacketing the further stack, for example eitherbefore or after the drawing of the further stack. The using of the drawnfurther stack to form an imaging fibre apparatus may comprise jacketingthe further stack. The method may comprise placing the further stack ina jacket tube, optionally with at least one further element, for examplepacking glass, and performing for example a heating process and/orcompression process and/or a further drawing process on the combinationof the further stack and jacket tube.

The plurality of rods may be substantially unjacketed when the drawingprocesses on the plurality of stacks are performed. The further stackmay be substantially unjacketed, or may be jacketed, when the drawing ofthe further stack is performed.

The selected arrangement may be such that, for each rod, thenearest-neighbour rods for said rod have different core sizes to saidrod.

For each stack, the arranging of the respective plurality of rods havingdifferent core sizes may comprise arranging the respective plurality ofrods such that nearest-neighbour rods have different core sizes.

The arranging may be such that no rod is adjacent to another rod havingthe same core size.

Arranging rods such that nearest neighbour rods have different coresizes may reduce cross-talk between cores. Reducing cross-talk mayimprove signal transmission through the optical fibre apparatus. Forexample, if the apparatus is used for imaging, imaging quality may beimproved.

The selected arrangement may be such that, for each rod, thenext-to-nearest neighbour rods for said rod have different core sizes tosaid rod.

Arranging the plurality of rods having different core sizes may comprisearranging the plurality of rods in a regular array.

The or each selected shape, for example the shape of a cross-section ofthe stack, may comprise a regular shape. The or each regular shape maycomprise at least one of a square, a rectangle, a rhombus, aparallelogram, a hexagon, a regular polygon. Each of the stacks may havesubstantially the same selected shape.

The drawn stacks may be stacked together such that each stack is in thesame orientation. The further stack may comprise a periodic arrangementof rods.

The stacking of the drawn stacks together to form a further stack maycomprise stacking the drawn stacks to substantially fill a desiredspace, such that there is substantially no gap between the drawn stacks.The drawn stacks may be arranged such that at least some adjacent rodsin different stacks are touching.

Each stack may have the same arrangement of rods. Each of the stacks mayhave substantially the same selected arrangement of rods.

Each drawn stack may comprise at least one unit cell. The further stackmay comprise a repeating arrangement of unit cells.

Stacking the drawn stacks of the at least one selected shape maycorrespond to a tiling of the selected shapes or shapes to substantiallyfill a plane. The tiling may be such as to form a repeating arrangementof rods of different sizes.

The use of stacks having at least one selected shape may result in easymulti-stacking. For example, a stack that is arranged as a square arraymay be drawn down to form a square unit cell, which may stack easilywith other square unit cells.

The stack may comprise an n×n square array of rods, where n is greaterthan or equal to 4.

Each stack may comprise a plurality of rows and a plurality of columns.For each stack, each row of the stack may comprise at least one rod ofeach of a plurality of different core sizes. Each column of the stackcomprises at least one rod of each of the plurality of different coresizes.

Using a square array or rectangular array may make it easy to arrangeany desired number n of different core sizes, for example by includingone or more rod having each of the different core sizes per row orcolumn. For example, it may be easier to arrange the different sizes ofrods in a square array than in a hexagonal array.

The plurality of different core sizes may comprise N different coresizes

Each stack comprise an N by N array of rods. For each stack, each row ofthe stack may comprise one rod of each of the N different core sizes.For each stack, each column of the stack may comprise one rod of each ofthe N different core sizes.

Each stack may comprise an array of yN columns by zN rows. For eachstack, each row of the stack may comprise y rods of each of the Ndifferent core sizes. For each stack, each column of the stack maycomprise z rods of each of the N different core sizes.

Five different core sizes may be denoted A to E in order of ascendingsize. The stack may comprise a square array having the followingarrangement of rods:

DACEB

CEBDA

BDACE

ACEBD

EBDAC.

The plurality of rods may comprise rods having at least five differentcore sizes. The arranging of the plurality of rods may be such thatnext-to-nearest neighbour rods have different core sizes.

The arranging may be such that no rod has a nearest neighbour ornext-to-nearest neighbour rod having the same core size.

Arranging the rods such that next-to-nearest neighbour rods havedifferent core sizes may further reduce cross-talk.

The plurality of rods may comprise rods having at least nine differentcore sizes. The arranging of the plurality of rods may be such thatnext-to-next-to-nearest neighbour rods have different core sizes. Thearranging may be such that no rod has a nearest neighbour,next-to-nearest neighbour or next-to-next-to-nearest neighbour rodhaving the same core size.

The stacking of the drawn stacks together to form a further stack maycomprise stacking the drawn stacks together to form a repeatingarrangement of unit cells.

Using a process in which a stack is drawn down and then further stackedmay allow an optical fibre apparatus having a large number of cores tobe formed in multiple stages. The stack may be configured for easystacking. For example, a square stack may easily stack with other squarestacks. In some circumstances, the use of a stack that has a simplegeometric shape may result in some tolerance for distortions, forexample tolerance for deformations resulting from drawing down.

The forming of the optical fibre apparatus may comprise performing atleast one further stacking and drawing of the drawn further stack.

For each stack, arranging the respective plurality of rods to form thestack may comprise positioning spacer elements between at least some ofthe respective plurality of rods. For example, the spacer elements maycomprise solid rods. The spacer elements may not be light-transmitting.

Each rod may comprises at least one of silica, Ge-doped silica, Fluorinedoped silica, boron doped silica, Aluminium doped silica, silicateglass.

The outer diameters of the plurality of rods of different sizes may bebetween 0.5 mm and 10 mm, optionally between 1 mm and 5 mm. The outerdiameters of the plurality of rods of different sizes may be greaterthan 0.1 mm, optionally greater than 0.5 mm, further optionally greaterthan 1 mm. The outer diameters of the plurality of rods of differentsizes may be less than 20 mm, optionally less than 10 mm, furtheroptionally less than 5 mm.

A width of the stack may be between 1 mm and 1000 mm, optionally between5 mm and 500 mm, further optionally between 10 mm and 100 mm. The widthof the stack may be greater than 1 mm, optionally greater than 5 mm,further optionally greater than 10 mm. The width of the stack may beless than 1000 mm, optionally less than 500 mm, further optionally lessthan 100 mm.

A numerical aperture of each rod may be less than 0.35, optionally lessthan 0.32, further optionally less than 0.3.

The optical fibre apparatus may comprise an imaging fibre apparatus. Theoptical fibre apparatus may comprise a coherent imaging fibre.

In a second aspect of the invention, which may be providedindependently, there is provided an optical fibre apparatus comprisingcores of different core sizes separated by cladding, wherein the coresof different core sizes are arranged to form a selected repeatingpattern of different core sizes.

The repeating arrangement may comprise a repeating arrangement of unitcells. Each unit cell may have the same arrangement of different coresizes.

Each unit cell may comprise a n×n square array of cores. n may be isgreater than or equal to 4.

Each unit cell may comprise a plurality of rows of cores and a pluralityof columns of cores. Each row may comprise at least one core of each ofa plurality of different core sizes. Each column may comprise at leastone core of each of the plurality of different core sizes.

The cores may be arranged such that for each core, the nearest-neighbourcores for said core have different core sizes to said core.

The cores may arranged such that for each core, thenext-to-nearest-neighbour cores for said core have different core sizesto said core.

The optical fibre apparatus may comprise a plurality of rows and aplurality of columns. Each row may comprise at least one core of each ofthe different core sizes. Each column may comprise at least one core ofeach of the different sizes.

The plurality of cores may comprise cores having at least five differentsizes. The cores may be arranged such that for each core thenext-to-nearest neighbour cores for said core have different core sizesto said core.

Each of the cores may have a core diameter between 1 μm and 100 μm. Eachof the cores may be configured to guide light in a transmissiondirection. The diameter of each core may be determined in a plane thatis substantially perpendicular to the transmission direction. The corediameter may comprise, for example, a maximum diameter, a minimumdiameter, or an average diameter.

A centre to centre spacing of the cores may be less than 100 μm,optionally less than 10 μm, further optionally less than 5 μm.

The optical fibre apparatus may be configured for use in imaging.

The optical fibre apparatus may be configured to transmit at least oneof visible light, infrared light, ultraviolet light.

An optical coupler may be coupled to the optical fibre apparatus and toa light source and/or light detector.

There may be provided a fibre assembly comprising: an optical fibreapparatus as claimed or described herein or formed using a method asclaimed or described herein; at least one further optical fibre and/orat least one capillary tube; and a package containing the optical fibreand the at least one optical fibre and/or at least one capillary. Thepackage may comprise, for example, a glass or polymer tube. The opticalfibre apparatus may comprise an imaging fibre apparatus.

The fibre assembly may comprise any combination of elements, for exampleany combination of imaging fibres, sensing fibres and/or supportelements. The fibre assembly may be formed from stacks, which maycomprise stacks of different types of element. For example, imagingstacks may be interspersed with sensor stacks. The fibre assembly may beformed from stacks having different shapes and/or sizes. For example,large imaging stacks may be interspersed with small sensor stacks.

The fibre assembly may further comprise an optical coupler configured tocouple the optical fibre apparatus to a light source and/or lightdetector.

The fibre assembly may further comprise a further coupler configured tocouple the or each further optical fibre to at least one sensingapparatus

The fibre assembly may further comprise a connector configured to couplethe or each capillary tube to a fluid insertion device, for example asyringe.

A distal end of the fibre assembly may be configured for insertion intoa human or animal subject.

There may also be provided an apparatus or method substantially asdescribed herein with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. For example,apparatus features may be applied to method features and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described, by way of non-limitingexamples, and are illustrated in the following figures, in which:—

FIG. 1 is a schematic illustration of an apparatus for drawing down afibre preform to form an optical fibre;

FIG. 2 is a flow chart illustrating in overview a method of anembodiment;

FIG. 3 is a schematic illustration of one preform drawn to fivedifferent sizes and stacked in a square stack;

FIG. 4 is a schematic illustration of the stack of FIG. 3 when drawndown and stacked again;

FIG. 5 is a schematic illustration of the stack of FIG. 4 when drawndown and stacked again and placed into a jacket tube;

FIGS. 6 a, 6 b and 6 c are scanning electron microscope (SEM) images ofthe cores of a coherent imaging fibre according to an embodiment;

FIG. 7 is a schematic illustration of a hexagonal stack of 331 rods;

FIG. 8 is a schematic illustration of a second stage of hexagonalstacking, where each hexagon represents a respective initial stack of331 rods as shown in FIG. 7 ;

FIG. 9 is a SEM image of a hexagonal array imaging fibre;

FIG. 10 is a schematic illustration of a 6×6 array of drawn squarestacks, where the square stacks are as shown in FIG. 3 ;

FIG. 11 is a SEM image of a fibre formed from the array of FIG. 10 whenrestacked and jacketed and drawn to fibre;

FIG. 12 is a USAF 1951 test target fluorescent image taken through ahexagonal array fibre at 520 nm to 600 nm;

FIG. 13 is a USAF 1951 test target fluorescent image taken through ahexagonal array fibre at 650 nm to 750 nm;

FIG. 14 is a USAF 1951 test target fluorescent image taken through asquare array fibre at 520 nm to 600 nm;

FIG. 15 is a USAF 1951 test target fluorescent image taken through asquare array fibre at 650 nm to 750 nm; and

FIG. 16 is a plot of fringe visibility measurements varying withwavelength for a square array fibre, a hexagonal array fibre, and acommercial imaging fibre made by Fujikura, Ltd.

FIG. 1 is a schematic illustration of a fibre drawing apparatus 10comprising heating elements 20 and a fibre pulling mechanism 22. Othercomponents of the fibre drawing apparatus have been omitted for clarity.FIG. 1 is not illustrated to scale.

In the present embodiment, the fibre drawing apparatus 10 is an item oftelecommunications equipment and is configured to control diameter of adrawn fibre to within microns. In other embodiments, any suitable fibredrawing apparatus may be used.

The fibre drawing apparatus 10 is configured to draw a fibre preform 30comprising a core region 32 and cladding region 34.

To draw the fibre preform 30, the fibre preform is pulled by the pullingmechanism in a direction indicated by arrow 40 (which in FIG. 1 isdownwards).

The fibre preform 30 is heated by heating elements 20 so that it becomessoft and may be drawn. The fibre preform 30 is pulled by the pullingmechanism 22 so that it increases in length and decreases incross-section. The output of the fibre pulling apparatus 10 is a rod 36having substantially the same ratio of core size to cladding size as theoriginal fibre preform 30, but a much smaller cross-section.

The ratio of core size to cladding size (for example, a ratio of corediameter to cladding diameter) may be referred to as a core to claddingratio. The cladding size may also be referred to as an outer size.

FIG. 2 is a flow chart illustrating in overview a method of forming anoptical fibre apparatus in accordance with an embodiment. The opticalfibre apparatus is a multiple core optical fibre apparatus. In thisembodiment, the optical fibre apparatus is a coherent imaging fibrecomprising 2025 imaging cores. In other embodiments, the optical fibreapparatus may be any suitable optical fibre apparatus comprising anynumber of cores, for example comprising hundreds, thousands, or tens ofthousands of cores.

At stage 50 of FIG. 2 , fibre drawing apparatus 10 is used to draw asingle type of fibre preform 30 to form rods of five different sizes36A, 36B, 36C, 36D, 36E. Individual core rods with low index claddingare drawn from the single type of fibre preform. The single type offibre preform comprises one or more pieces of fibre preform each havingthe same material composition, core diameter and cladding diameter.

In the present embodiment, fibre preform 30 is a multimode telecomsgrade preform which is formed from silica. The fibre preform 30 isdesigned for mass production. The fibre preform 30 is graded index witha parabolic graded index profile.

Fibre preform 30 has an outer diameter of 30 mm and a length of 1 metre.Fibre preform 30 comprises a cladding region 34 having a refractiveindex of 1.46 and a core region 32 having a peak refractive index of1.49. The diameter of the core region 32 is 23 mm. The core to claddingratio of the fibre preform 30 is 0.72. In other embodiments, anysuitable fibre preform may be used.

The fibre preform 30 may be considered to be an off-the-shelf componenthaving a standard size.

The fibre preform 30 is drawn down to five different sizes using fibredrawing apparatus 10. For example, fibre drawing apparatus 10 may drawdown a length of fibre preform 30 to a first size to form a firstplurality of rods 36A by operating the fibre drawing apparatus 10 at afirst speed and cutting off 1 metre lengths of rod to form the firstplurality of rods 36A. In other embodiments, any lengths of rods may beused.

Fibre drawing apparatus 10 may then draw down a further length of fibrepreform 30 to a second size to form a second plurality of rods 36B byoperating the fibre drawing apparatus 10 at a second speed, and cuttingoff 1 metre lengths of rod to form the second plurality of rods 36B.Apparatus 10 may draw down further lengths of fibre preform 30 to third,fourth and fifth sizes to form third, fourth, and fifth pluralities ofrods 36C, 36D, 36E respectively by operating the fibre drawing apparatus10 at third, fourth and fifth speeds. In other embodiments, any lengthsof rod may be used, and any number of pieces of fibre preform may beused to form the rods.

Different sizes of rods 36A, 36B, 36C, 36D, 36E may be produced from thesame preform by operating the fibre drawing apparatus 10 at differentspeeds, without making any other changes to the preform, for examplewithout an additional jacketing stage.

In other embodiments, any suitable drawing process and drawing apparatusmay be used. For example, a fibre drawing process may be as described atpage 8 of An Introduction to Fiber Optic Imaging, Schott North America,Second Edition, Schott, 2007.

In the description below, references to rods having different core sizesand/or outer sizes refer to rods having different core sizes and/orouter sizes in a direction perpendicular to a length of the rod, forexample rods having different core and/or outer diameters, and/ordifferent core and/or outer cross-sectional areas.

Each plurality of rods 36A, 36B, 36C, 36D, 36E has a different outerdiameter. Each plurality of rods 36A, 36B, 36C, 36D, 36E has a differentcore diameter. Since the rods 36A, 36B, 36C, 36D, 36E are made from thesame fibre preform 30, every rod 36A, 36B, 36C, 36D, 36E hassubstantially the same core to cladding ratio.

In the present embodiment, the outer diameters of the pluralities ofrods range between 2.23 mm and 3.17 mm. The outer diameters and corediameters vary by around 8% from a central value. From smallest tolargest, the sizes are −16%, −8%, 0%, +8% and +16% when compared with acentral value. In other embodiments, different outer diameters may beused. Any appropriate range of sizes may be used.

In the present embodiments, the first plurality of rods 36A each have anouter diameter of 2.23 mm. The second plurality of rods 36B each have anouter diameter of 2.52 mm. The third plurality of rods 36C each have anouter diameter of 2.74 mm. The fourth plurality of rods 36D each have anouter diameter of 2.95 mm. The fifth plurality of rods 36E each have anouter diameter of 3.17 mm.

In the present embodiment, five different sizes of rods 36A, 36B, 36C,36D, 36E are drawn from fibre preform 30. In other embodiments adifferent number of sizes of rods may be drawn. For example, n differentsizes of rod may be drawn, where n is at least 3. In the presentembodiment, all five different sizes of rods are drawn from the sametype of fibre preform. In other embodiments, the different sizes of rodsmay be drawn from different fibre preforms. Any appropriate rod that isconfigured for light transmission may be used, for example any rod thatis configured to transmit infrared, visible and/or ultraviolet light.

At stage 52 of FIG. 2 , five of each size of rod 36A, 36B, 36C, 36D, 36E(i.e. 25 rods in total) are stacked in a 5×5 square array to form astack of rods which may be referred to as a primary stack 70. We notethat the terms arranging and stacking may be used interchangeably todescribe the vertical arrangement of elements.

FIG. 3 is an illustration of a cross-section of the primary stack 70.Although in the present embodiment the rods are stacked together withoutthe inclusion of any further rods, in other embodiments spacer rods maybe included between at least some of the rods 36A, 36B, 36C, 36D, 36E.The spacer rods may also be referred to as spacer elements. The spacerrods may not comprise cores, and may not be configured to transmitlight.

Each row of the primary stack 70 comprises 5 rods 36A, 36B, 36C, 36D,36E, each having a different core size, such that all five core sizesare represented in the row. Similarly, each column of the primary stack70 comprises 5 rods 36A, 36B, 36C, 36D, 36E, each having a differentcore size, such that all five core sizes are represented in the row. Therods are stacked such that each rod has a different core size from itsnearest neighbours. Each rod also has a different core size from itsnext-to-nearest neighbours. In the present embodiment, no rod is anearest neighbour or next-to-nearest neighbour of another rod having thesame core size.

In the present embodiment, if the rods 36A, 36B, 36C, 36D, 36E arereferred to by letters A to E (A to E being in order of ascending size),the arrangement of the rods 36A, 36B, 36C, 36D, 36E in the primary stackmay be represented as an array of letters as shown below, where rows andcolumns of letters represent rows and columns of rods:

DACEB

CEBDA

BDACE

ACEBD

EBDAC.

The 5×5 array comprises 5 rods of each of the 5 sizes. Each differentrod size appears once in each row or column of the square stack. Noadjacent rod sizes are the same. The different sizes of rods 36A, 36B,36C, 36D, 36E are arranged such that, even though the individualelements of the primary stack 70 differ in size, the primary stack 70 asa whole is substantially square. The primary stack may be considered tohave a selected shape, which in this embodiment is a square. In otherembodiments, the selected shape may be any regular shape, for example arectangle, parallelogram, rhombus or hexagon. The selected shape maycomprise, for example, any regular polyhedron. The selected shape may besuch that a primary stack having the selected shape may be stacked withother primary stacks having the selected shape and/or other primarystacks having one or more further selected shapes.

From FIG. 3 , it may be seen that each individual row and/or column ofthe primary stack 70 may not be entirely straight due to the sizedifferences between the rods, but that it may be possible to draw asubstantially square outer boundary around the primary stack 70.

Once the rods 36A, 36B, 36C, 36D, 36E have been stacked as a primarystack 70 as shown in FIG. 3 , the ends of the rods 36A, 36B, 36C, 36D,36E are fused to inhibit relative movement between the rods 36A, 36B,36C, 36D, 36E, keeping the primary stack 70 of rods arranged as asquare. In the present embodiment, the rods 36A, 36B, 36C, 36D, 36E aretaped using PTFE tape and fused at both ends by hand. In otherembodiments, the rods In the present embodiment, the rods 36A, 36B, 36C,36D, 36E may be held together in the square stack 70 in any suitablemanner.

Although only one stack 70 is shown in FIG. 3 , in practice many stacks70 are formed at stage 52. Each array is a 5×5 array of rods 36A, 36B,36C, 36D, 36E as shown in FIG. 3 . In the present embodiment, the totalnumber of stacks 70 that is made at stage 52 is 81.

At stages 54 and 56 of FIG. 2 , the primary stack 70 which forms auniform square is drawn down and stacked again as described below.

At stage 54, each primary stack 70 is drawn down using fibre drawingapparatus 10, or using a further fibre drawing apparatus (not shown).During the drawing process, the rods 36A, 36B, 36C, 36D, 36E are heatedand become fused to each other at the points at which they touch.However, interstitial air gaps remain between the rods 36A, 36B, 36C,36D, 36E.

The primary stack 70, once drawn down, may be considered to form asecondary unit cell 72. The rods 36A, 36B, 36C, 36D, 36E may beconsidered to be non-uniformly sized primary unit cells in amulti-stacking process. Stages 52 and 54 may be considered to create auniformly sized secondary unit cell from non-uniformly sized primaryunit cells 36A, 36B, 36C, 36D, 36E.

Secondary unit cell 72 has a much smaller cross-section than the primarystack 70 from which it is drawn down. However, secondary unit cell 72remains substantially square after drawing down. The rods 36A, 36B, 36C,36D, 36E maintain their square arrangement.

In the present embodiment, the primary stack 70 shown in FIG. 3 has aside length of 14 mm. The secondary unit cell 72 obtained from theprimary stack 70 has a side length of 2.5 mm.

At stage 56, 9 secondary unit cells 72 are stacked in a 3×3 array, whichmay be referred to as a secondary stack 74. Secondary stack 74 isillustrated in cross-section in FIG. 4 . The extent of the individualsecondary unit cells 72 in the secondary stack 74 is indicated by dashedlines. The stacking of the secondary unit cells 72 forms a repeatingarrangement of rods. Each secondary unit cell 72 is formed from a 5×5array of rods, so the secondary stack 74 comprises a repeatingarrangement of rods that repeats every 5 columns and every 5 rows.

The secondary unit cells 72 are taped and fused at both ends. In otherembodiments, the secondary unit cells 72 may be held together in anysuitable manner.

FIG. 4 shows one secondary stack 74, which is formed from 5 secondaryunit cells 72. In practice, further secondary unit cells 72 are alsostacked together to form further secondary stacks 74. A total of 9secondary stacks 74 are formed.

The uniform size and shape of the secondary unit cells 72 (which in thisembodiment are square) may allow the secondary unit cells 72 to beeasily stacked. The secondary stack 74 may be a stack of identical ornear-identical components (secondary unit cells 72). The secondary unitcells 72 may be considered to be tiled such that they fill space.

In other embodiments, the secondary unit cells may have any regulargeometric shape that may be stacked together, for example a rectangle, aparallelogram, a rhombus, or a hexagon.

In further embodiments, each secondary unit cell has an interlockingshape (for example, a shape that may be considered to resemble a jigsawpiece). At stage 56, the secondary unit cells are arranged such thatthey interlock.

In some circumstances, there may be some deformation of the primary orsecondary stacks when they are drawn. For example, there may be sometwisting of the stacks. In some circumstances, some types of distortionmay prevent such an interlocking unit cell from successfullyinterlocking with other unit cells. It may be the case that the use of aregular shape that is not interlocking (for example, a square orhexagon) may result in greater tolerance to distortions.

In the present embodiment the secondary unit cells 72 are stacked in a3×3 square array. In other embodiments, any number of secondary unitcells 72 may be stacked together. Any suitable stack of secondary unitcells 72 may be used. The stack of secondary unit cells 72 may have anyregular geometric shape.

At stages 58 to 62, the new uniform square stack (secondary stack 74) isdrawn down and stacked again, placed in a jacket tube 80 and packingglass 82 added to hold the structure. Each of stages 58 to 62 is nowdescribed in detail.

At stage 58, each secondary stack 74 is drawn down using fibre drawingapparatus 10, or a further drawing apparatus. During the drawingprocess, the secondary unit cells are heated and rods of each secondaryunit cell 72 become fused to neighbouring rods from other secondary unitcells 72 at the points at which they touch. Interstitial air gaps remainbetween rods of different secondary unit cells 72, and between rodswithin each secondary unit cell 72.

The secondary stack 74, once drawn down, may be considered to form atertiary unit cell 76. Tertiary unit cell 76 has a uniform shape, whichmay make it easy to stack together. In the present embodiment, thetertiary unit cell 76 has a side length of 4.5 mm.

At stage 60, 9 tertiary unit cells 76 are stacked together in a 3×3arrangement as shown in FIG. 5 . In FIG. 5 , dashed lines indicate theextent of each tertiary unit cell 76. In other embodiments, any suitableshape of stack may be used. Any number of tertiary unit cells 76 may bestacked together.

At stage 62, the stack of 9 tertiary unit cells 76 is placed in a jackettube 80. Packing glass 82 is added between the jacket tube and thetertiary unit cells 82 to hold the tertiary unit cells 76 in their 3×3arrangement within the jacket tube 80. The packing glass 82 comprises aplurality of solid glass rods that are not configured to transmit light.The solid glass rods are placed around the square stack of tertiary unitcells 76 to fill space between the square stack and the round jackettube 80. The tertiary unit cells 76, jacket tube 80 and packing glass 82may be considered to form a final assembly 78.

At stage 64, the final assembly 78 is drawn down using fibre drawingapparatus 10, or a further drawing apparatus. A vacuum attachment isused to suck out air from gaps between the cores, removing the gapsbetween the rods.

The drawing down process of stage 64 results in an optical fibreapparatus, which in this embodiment is a coherent imaging fibre. Thecoherent imaging fibre may be configured to transmit visible, infraredand/or ultraviolet light. The coherent imaging fibre may be consideredto form an array of light guiding elements (the light guiding elementsin this case are the cores of the rods that have been drawn to form thecoherent imaging fibre), arranged in a repeating arrangement of lightguiding elements. As described above, the repeating arrangement of lightguiding elements having different sizes may reduce cross-talk betweenthe light guiding elements. For example, no light-guiding element mayhave a nearest- or next-to-nearest light guiding element of the samesize.

FIGS. 6 a, 6 b and 6 c each show scanning electron microscope images ofcores of a coherent imaging fibre according to an embodiment. FIG. 6 ashows the cores at a magnification of ×1,300. FIG. 6 b shows the coresat a magnification of ×2,500. FIG. 6 c shows the cores at amagnification of ×10,000.

In the present embodiment, the cores are drawn such that all the coresin the coherent imaging fibre are multimode. The cores are drawn suchthat the smallest core 36A does not become single mode. In otherembodiments, the cores may be drawn such that all of the cores in theimaging fibre are single mode.

In the present embodiment, a spacing between the cores in the coherentimaging fibre is around 3.5 μm. For comparison, a spacing between coresin a multicore telecommunications fibre may be around 10 μm. The spacingbetween cores may be a spacing in a plane that is substantially parallelto a direction of light transmission.

In further embodiments, a diameter of each of the cores in the coherentimaging fibre may be, for example, between 1.5 μm and 10 μm. Thediameter of the cores may be measured in a plane that is substantiallyperpendicular to a direction of light transmission. In some embodiments,the cores in the coherent imaging fibre may not be circular. If thecores are not circular, any diameter of the cores may be measured, forexample a largest diameter or smallest diameter.

When considering core sizes, it may be taken into account that smallercores may be placed closer together than larger cores. Placing corescloser together may improve imaging resolution.

In the present embodiment, the coherent imaging fibre is configured totransmit light in a range of wavelengths from 400 nm to 850 nm. In otherembodiments, the coherent imaging fibre may be configured to transmitlight at any suitable wavelengths, for example at wavelengths between350 nm and 2000 nm.

In the present embodiment, an outer diameter of the fibre is 550 μm.However in other embodiments the outer diameter of the fibre may beincreased or decreased by adding or subtracting cores. Adding orsubtracting cores may change the field of view of the fibre.

In some circumstances, fibres may start to become not flexible at adiameter of over 1 mm. However, image guiding or manipulating rigid rodsmay be formed. In some circumstances, fibres of less than 80 μm indiameter may contain too few cores to provide good imaging performance.

In the present embodiment, the coherent imaging fibre is between 1 metreand 5 metres long. In other embodiments, any suitable length of imagingfibre may be used. In many embodiments, for example in medical imagingembodiments, a length of a few metres may be appropriate. This lengthmay be compared with that of fibres used for telecommunicationsapplications, which may be kilometres long. In some circumstances, thecores in an imaging fibre according to an embodiment may be closertogether than cores in a conventional telecommunications fibre.

In the present embodiment, the coherent imaging fibre resulting from theprocess of FIG. 2 is packaged with a plurality of sensing fibres and acapillary tube to form a multi-functional fibre apparatus. The coherentimaging fibre, sensing fibres and capillary tube are placed within afurther glass or polymer tube, which may be referred to as a package.The further glass or polymer tube may be shorter than the coherentimaging fibre, sensing fibres and capillary tube. The package containsthe coherent imaging fibre, sensing fibres and capillary tube, which arefixed in place using epoxy.

One end of the resulting fibre apparatus, which may be referred to asthe distal end, is filled with epoxy and then polished.

At the other end of the fibre apparatus, which may be referred to as thedistal end, the coherent imaging fibre, sensing fibres and capillarytube extend beyond the further glass or polymer tube. An opticalcoupler, for example an FC connector, is attached to the distal end ofthe coherent imaging fibre and is used to couple the coherent imagingfibre to an optical instrument, for example to a device comprising anoptical source and optical detector.

A distal end of the each of the sensing fibres may be coupled to asensing apparatus, for example an optical instrument and/orspectrometer.

A connector is attached to a distal end of the capillary tube. Theconnector allows a syringe to be coupled to the capillary tube in orderto introduce substances into the capillary tube, for example tointroduce fluorescent probes.

In other embodiments, the optical fibre apparatus may be packaged withany suitable further fibre and/or capillary tube. In some embodiments,the optical fibre apparatus, further fibre and/or capillary tube arefabricated separately and then packaged together. In other embodiments,the further fibre and/or capillary tube are fabricated in the samefabrication process as the optical fibre apparatus. For example, thefurther fibre and/or capillary tube may be drawn down in the samedrawing process as rods of the optical fibre apparatus.

In use, a distal end of the multi-functional fibre assembly ispositioned inside the body of a human or animal subject, for exampleinside the lung of a patient. Any suitable method of deploying themulti-functional fibre assembly may be used. For example, themulti-functional fibre assembly may be inserted through the workingchannel of the bronchoscope and deployed from the working channel intothe distal lung.

The capillary tube is used to introduce substances into the distal lung,for example to introduce fluorescent probes to facilitate imaging. Theimaging fibre is used to image the distal lung. The sensing fibres areused to sense signals from the distal lung. The imaging fibre and/orsensing fibres may deliver light from the distal lung to a spectrometerconfigured to perform spectroscopy of the delivered light.

The imaging fibre may be used to project an image using lenses. Theimaging fibre may be used to project an image onto a correspondingsensor.

It may be considered that the method of FIG. 2 provides a fabricationtechnique creating a uniform sized secondary unit cell 72 fromnon-uniform sized primary unit cells (the differently-sized rods 36A,36B, 36C, 36D, 36E) in a multi-stacking process. The technique comprisespacking n non-uniform elements in an n×n square array such that each rowand column contains the same variation of rod sizes resulting in asquare unit cell for the next multi-stacking phase.

In the present embodiment, n=5. The preform is drawn to form 5 differentsizes of rod. The 5 different sizes of rod are packed in a 5×5 squarearray such that each row or column contains one of each size of rod.

The method of FIG. 2 allows for a single preform to be used to produceall the cores of the multiple core optical fibre apparatus. The methodof FIG. 2 may allow for the size of the cores to be easily varied. Byadding extra variations in core size, cores of the same size may be keptsignificant distances apart. The use of a single preform may lead tolower costs and/or a simpler process than methods in which differentpreforms with different core to cladding diameter ratios are used.

In the method of FIG. 2 , one conventional telecommunications preform isused to form the optical fibre apparatus without the need for jackettubes. In some methods, after drawing rods from the preform, the rodsmay be placed into different jacket tubes. The jacket tubes may then bedrawn again by making a stack, which may add significant time to thefabrication process. Since the method of FIG. 2 does not use such ajacketing step, the method FIG. 2 may be faster than methods that do usesuch a jacketing step.

By using the method of FIG. 2 , a significant reduction in fabricationtime and/or material costs and/or production costs may be obtained.Telecommunications preforms may be cheap and widely available. The useof telecommunications preforms (whether or not a single type of preformis used) may reduce costs. Telecommunications preforms may have a lowernumerical aperture than some preforms that are used for imaging. The useof different core sizes to reduce cross coupling may allowlower-numerical aperture materials to be used than would be possible ifall the core sizes were the same.

Using the multi-stacking method described above with reference to FIG. 2, primary stacks can be drawn down to smaller rods and this secondaryunit cell restacked to easily increase the number of cores in a fibre(instead of stacking large number of rods initially, for examplestacking tens of thousands of rods).

It may be the case that coupling in multicore fibres is worse if thecores are identical; if there is low contrast between the core andcladding of an individual rod; and/or if the wavelength used isrelatively long. In the embodiment of FIG. 2 , separation of identicalcores is used to reduce coupling between cores of a multicore fibre. Theseparation of identical cores that are formed from the same preform mayprovide a cheap and effective method of reducing coupling between cores.

In some circumstances, the use of different sizes of cores may mean thatit is not necessary to use other methods, for example interstitialelements, in order to reduce cross-talk between cores.

The differences in core size between the different rods 36A, 36B, 36C,36D, 36E may not have a significant impact on imaging performance. Insome embodiments, an imaging performance of an imaging fibre made usingthe method of FIG. 2 may be similar to an imaging performance achievedusing heavily-doped Ge-doped glass having random core size variation.However, the imaging fibre made using the method of FIG. 2 may becheaper than imaging fibre formed of heavily-doped Ge-doped glass havingrandom core size variation.

The use of mass produced materials shared by the telecommunicationsindustry may reduce imaging fibre cost. In an imaging fibre, it may bedesired to place individual cores of the fibre as close together aspossible, but core to core spacing may be limited by the cross couplingof light between the cores. By reducing cross-coupling, the method ofFIG. 2 may allow cores to be placed close together, which may result ina fibre capable of transmitting a high resolution image.

The method of FIG. 2 may be used to develop a low-cost imaging fibrewhich may be packaged into a robust device such that it may bedisposable after a single use. Producing a low-cost disposable imagingfibre may eliminate the need for sterilization procedures between uses,and therefore may eliminate any degradation of the imaging fibre thatmay occur during sterilization. Contamination crossover between patientsmay be prevented. Clinical workflow may be significantly improved.

In some embodiments, the imaging fibre made using the method of FIG. 2is a flexible imaging fibre. In some embodiments, the imaging fibre is arigid fibre, for example a rigid imaging rod.

The imaging fibre, for example the rigid imaging rod, may be used toproject an image onto a sensor array. The regular array of cores in theimaging fibre may be used in projecting the image onto a sensor array.The sensor array may be a regular array of sensors, for example a squarearray. In some circumstances, the regular array of the imaging fibre maybe preferable for projecting onto a sensor array when compared to animaging fibre in which different sizes of cores are randomly arranged.

In some circumstances, the fabrication method of FIG. 2 may ensure analmost arbitrary separation of cores of the same size with the initialuse of one preform. Any suitable number of different core sizes may beused.

In one embodiment, a fibre preform is drawn to form rods having 4different core sizes. The rods are stacked in a 4×4 square array inwhich each row or column contains one of each size of rod. By using atleast 4 different sizes of rod, it may be possible to obtain anarrangement of rods in which each rod has no nearest neighbour that hasthe same size.

In the embodiment described above with reference to FIGS. 2 to 5 , thepreform 30 is drawn to form rods of 5 different sizes. The rods arestacked in a 5×5 square array in which each row or column contains oneof each size of rod. By using at least 5 different sizes of rod, it maybe possible to obtain an arrangement of rods in which each rod has nonearest neighbour or next-to-nearest neighbour that has the same size.

In some embodiments, the preform is drawn to form rods of 9 differentsizes. The rods are arranged as a square array of rods in which each rowor column contains one of each size of rod. By using at least 9different sizes of rods, it may be possible to obtain an arrangement ofrods in which each rod has no nearest neighbour, next-to-nearestneighbour, or next-to-next-to-nearest neighbour that has the same size.

In further embodiments, any number of different sizes of rods may beused. In some circumstances, the use of a greater number of sizes of rodmay result in more distortion of the stack when it is drawn. There maybe a trade-off between spacing of identical cores against distortioneffect, since more sizes of core may result in a greater spacing ofidentical cores but also in increased distortion.

The rods may be arranged in any suitable manner, for example in a squarearray or hexagonal array. The rods may be arranged to form any one ormore selected shapes. For example, rods may be arranged to form a stackhaving the shape of a square, rectangle, rhombus, parallelogram orhexagon.

Any number of rods may be stacked to form a primary stack (which isdrawn to form a secondary unit cell). Any number of secondary unit cellsmay be stacked to form a secondary stack (which may be drawn to form atertiary unit cell). Any number of tertiary unit cells may be stacked toform a tertiary stack (which may be drawn to form an imaging fibre).

Any unit cell may be formed by stacking different sized rods to form anyselected shape, then stacking those regular shapes in a periodic latticeof those selected shapes. In some embodiments, two or more differentselected shapes may be stacked. For example, a square stack may beformed that comprises one unit cell (for example, one secondary unitcell) and a rectangular stack may be formed that comprises two unitcells. The rectangular stack may have the size, and the rod arrangement,of two of the square stacks. The square and rectangle may be stackedtogether to form a further stack.

In one embodiment, an imaging fibre having 8000 cores is formed bystacking 5×5 rods to form a primary stack; stacking 6×6 secondary unitcells to form a secondary stack; and stacking 3×3 tertiary unit cells toform a tertiary stack, which is drawn to form the imaging fibre.

In further embodiments, any number of multi-stacking iterations may beused. For example, the rods may be drawn down twice, three times, fourtimes, or five times.

Any suitable materials may be used to form the optical fibre apparatus.For example, any suitable preform may be used. The optical fibreapparatus may comprise, for example, silica, Ge-doped silica, Fluorinedoped silica, boron doped silica, Aluminium doped silica, or silicateglass. In some circumstances, silicate glasses may be used to get veryhigh index contrasts, for example Schott glasses SF6 and LLF1.

Although embodiments comprising imaging fibres are described above, inother embodiments any suitable multiple core optical fibre apparatus maybe formed. The multiple core optical fibre apparatus may be used for anysuitable purpose.

In the embodiments described above, the rods have different outer sizes.In other embodiments, an optical fibre apparatus may be formed usingrods that have substantially the same outer size, but different coresizes. The use of different core sizes may reduce cross-coupling evenwhen the rods are of the same size.

In some embodiments, different preforms are obtained (for example,different commercial preforms) and are drawn into similarly-sized rods.The different preforms have different core to cladding ratios.Therefore, once drawn to the same outer size, the rods have differentcore sizes. The rods are stacked into a regular array, for example asquare or hexagonal array. The arrangement of the rods is such that eachrod does not have any nearest-neighbour rods that have the same coresize.

In other embodiments, a single type of preform is obtained. The preformis then jacketed with further cladding of one or more different sizes,to create preforms having different core to cladding ratios. Thedifferent preforms are then drawn into rods of the same size, stackedand drawn.

We now describe a method to fabricate using high quality imaging fibresusing a single multimode telecommunications preform available fromDraka-Prysmian. (0M1 PCVD rod). Our techniques involve multi-stackingarrays of different sized cores such that no two adjacent cores are thesame size.

In our first fibre we jacket three different sizes of rods drawn fromour preform and stack them in a hexagonal array. In our second fibredemonstrate a technique to achieve low cross coupling over a broadwavelength range by drawing the preform down to rods of various sizesand stacking these rods in a square array. The distribution of the rodsin the square array is such that when it is drawn down it forms auniform square stack which can then be easily restacked multiple timesin order to form an imaging fibre of many thousands of cores. This mayeliminate the need to jacket and re-draw rods to form different sizedcores making this technique economical and rapid in comparison to ourhexagonal fabrication method.

The first fibre is a hexagonal array imaging fibre. The core materialfor our first fibre was derived from a commercial preform manufacturedfor telecoms applications. It had a graded index germanium doped coresurrounded by a thin pure silica jacket. The core-cladding diameterratio of the preform was 0.74 with a peak refractive index contrastcorresponding to an NA of 0.3.

The preform was drawn down to rods of three different sizes. Two sizesof the rods were jacketed in pure silica tubes with two different innerto outer diameter ratios, and the rest remained unjacketed. The jacketedrods were then drawn down again to form rods all of which had an outerdiameter of 1 mm but with three different core diameters. The uniformrods could now be stacked in a hexagonal array of 331 where no twoneighbouring rods had the same core diameter. FIG. 7 shows the layout ofour initial stack of 331 rods. Germanium doped regions are illustratedin grey and pure silica regions are white.

The top end of the stack was wrapped in PTFE tape to hold it in placeand allow it to be gripped in the chuck of a fibre drawing tower.Smaller sections of PTFE tape were also wrapped around the stack atdifferent points in order to hold it in place. The stack was drawnunjacketed down to canes with each section of PTFE tape removed beforeit reached the furnace. FIG. 8 shows the layout of our second stagestack, where each hexagon represents the initial stack of 331 cores.

To form a final fibre, 37 of the canes were stacked and jacketed in apure silica tube. This stack was then drawn to fibre with an outerdiameter of 525 μm using a vacuum to collapse the interstitial spaces. Ascanning electron micrograph of the final fibre can be seen in FIG. 9where the insert shows a higher magnification in order to see the corepattern. Doped regions appear lighter in the SEM. FIG. 9 shows amagnified section of the core pattern.

The final core diameters in the fibre were 2.78 μm, 2.45 μm and 2.12 μmwith a centre to centre separation of 3.71 μm. There were 12,247 coresin total in the final fibre.

Our second imaging fibre was a square array imaging fibre. The secondimaging fibre was formed from a graded index preform having the sameparameters to that used to make the first fibre. A stable stack was madefrom rods drawn to different outer diameters, so that there was no needfor a jacketing stage.

The fabrication method uses N different sized elements stacked in an N×Narray to form a uniform square element. Once the uniform square elementhas been formed it is easily stacked multiple times to build up a largearray of cores. The stacked squares can easily be drawn down again andrestacked in order to easily build up very large arrays.

A three stage process was used. Firstly a 5×5 array of 5 different rodsizes (2.23 mm, 2.52 mm, 2.74 mm, 2.95 mm and 3.17 mm) was stacked in asquare stacking jig such that each size only appears in each row orcolumn once. The end view of this stack was as shown in FIG. 3 . FIG. 3is a representation of an initial stack in which grey indicatesgermanium doped core regions and white indicates the pure silicacladding regions.

The ends of the stack were fused together using a hydrogen torch andPTFE tape was wrapped around the length at several points in order tohold the stack in place. The stack was fed into the furnace and drawndown to 2.5 mm sided squares, the PTFE tape being unwound before itreached the furnace. This process generated a set of square unitelements with a similar cross section to that shown in FIG. 3 .

The unit squares were restacked with the same orientation in a 6×6array. The second stacking stage of the process is shown in FIG. 10 .The ends were again fused and the central region held in place with PTFEtape. This stack was drawn to 4.5 mm sided squares and restacked in a3×3 array.

The final stack was placed into a jacket tube with pure silica packingrods around the outside and drawn to canes under a vacuum to remove theinterstitial gaps. Finally the canes were drawn to fibre.

An SEM image of the final fibre can be seen in FIG. 11 , in which dopedcores appear lighter. The core diameters in the fibre were between 2 and3 μm with 3 to 4 μm centre to centre separations depending on theparticular pairs of core sizes. There were 8,100 cores in total in thefinal fibre. The outer diameter of the fibre was 550 μm with an imagingsquare size of 450 μm along the diagonal. A scalloped appearance of theedge of the imaging region is due to the presence of glass packingtubes.

To compare the performance of our hexagonal array imaging fibre of FIGS.7 to 9 and the square array imaging fibre of FIGS. 3, 10 and 11 weperformed two tests. The first was to acquire fluorescence images of1951 USAF test targets, and the second was to transmit a fringe patternand measure the degradation of the fringe visibility with wavelength.

An endoscopic fluorescence imaging system was built in order to obtaintest target images which could be used to determine the resolution ofthe square array fibre and of the comparison fibre at differentwavelengths.

A supercontinuum source filtered to two excitation bands (420 nm to 510nm and 600 nm to 650 nm) was used as a light source for our experiment.The filtered excitation light passed through a dichroic beam splitterand was coupled to the square array fibre (or to the comparison fibre)through an aspheric lens with an NA of 0.5.

The USAF 1951 targets were imaged at zero working distance from thedistal end of the square array fibre (or the comparison fibre) witheither a green fluorescent or red fluorescent slide placed behind them.Light emerging back out of the proximal end of the square array fibre(or the comparison fibre) was imaged onto a CCD camera after passingthrough the dichroic beam splitter and a second collection filter withtwo wavelength bands, 520 nm to 600 nm (green band) and 650 nm to 750 nm(red band). These wavelength ranges for collection were chosen to be inin the range of several reported chemical imaging probes which have thepotential to be used to indicate the presence of bacterial or fungalpathogens. Using a two colour (wavelength band) system may allow for thedetection of multiple biological targets through the same imagingsystem.

Images of the USAF 1951 test targets taken at two wavelength bands canbe seen in FIGS. 12 to 15 . FIG. 12 shows the hexagonal array fibre at520 nm to 600 nm. FIG. 13 shows the hexagonal array fibre at 650 nm to750 nm. FIG. 14 shows the square array fibre at 520 nm to 600 nm. FIG.15 shows the square array fibre at 650 nm to 750 nm.

In the green band images for both fibres (FIG. 12 and FIG. 14 ), severalof the larger elements of group 7 are discernible down to element 4 inthe hexagonal array fibre and element 2 in the square array fibre. (Theelements have line widths of 2.76 μm and 3.10 μm respectively.) Theseare comparable sizes to the core to core separations, indicating thatvery little light is coupling from an illuminated core into itsneighbour in both fibres. In the red band (FIG. 13 and FIG. 15 ) theimage contrasts are both degraded due to core to core coupling,principally between higher order modes which are visible in the darkregions. However in our square array imaging fibre the larger elementsof group 7 are still discernible down to element 2 albeit with reducedvisibility compared to the images taken in the green band.

A second characterization method is a quantitative method developed tomeasure the global effects of core to core coupling and accuratelycharacterize the performance of imaging fibres, using a method describedin H. A. Wood, J. M. Stone, K. Harrington, T. Birks, and J. C. Knight,“Quantitative characterisation of endoscopic imaging fibres” inConference on Lasers and Electro-Optics, OSA Technical Digest (2016)(Optical Society of America, 2016), paper SM4P.6. The technique may beconsidered to be similar to the measurement of the modulation transferfunction in imaging systems relying on quantifying the degradation invisibility of a transmitted fringe pattern as it passes through animaging fibre.

FIG. 16 shows fringe visibility measurements varying with wavelength forour hexagonal array fibre (represented by triangles) and square arrayfibre (represented by circles) and a commercial imaging fibre, which isa section of FGIH-30-650s fibre from Fujikura Ltd (represented bycrosses). Transmission is shown for wavelengths between 500 nm and 700nm. The visibility is measured after transmission through approximately90 cm of fibre in each case. Error bars represent the standard deviationof a series of data points taken at a single wavelength, adjusting andthen realigning the system between each measurement.

Two arms of an interferometer were interfered at a known angle toproduce a vertical fringe pattern of known separation. The fringepattern was transmitted down the test fibre and the visibility measuredat the output on a CCD camera. Using a supercontinuum and amonochromator as an illumination source for our interferometer we wereable to measure the transmitted fringe visibility with wavelength. Ourinterferometer was set up with an angle of 2.29° to give 15 μm fringesat 600 nm wavelength. The fringe separation varies with wavelength dueto diffraction and the corresponding variation can be seen on the topaxis of FIG. 16 . The fringe pattern was formed on the end of −90 cmsections of our fibres and the emerging pattern was imaged onto a CCDcamera mounted on a goniometer in order to align the fringe pattern andCCD array. The output coupling was via a 0.5 NA aspheric lens.

In an ideal fibre, a fringe pattern would not lose any visibility as itpasses down the length. However, core-to-core coupling reduces thecontrast in the image and hence the measured visibility.

The FGIH-30-650s fibre is common in commercial systems specified overour wavelength band. The centre to centre core separation of this fibrewas measured to be around 3.5 μm, with core diameters of between 1.7 μmand 2.1 μm and NA reported to be 0.4. The cores in the fibre appeared tohave a random arrangement.

The fringe visibility of our hexagonal array fibre at short wavelengthsis highest up to a wavelength of 550 nm but its performance begins todegrade as the wavelength increases. As there are only three core sizesin this fibre we attribute this degradation in fringe visibility tolight coupling to the identical next nearest neighbour cores.

This is reinforced in the test target images where it can be seen inFIG. 13 that one particular size of core is coupling strongly into darkregions in a higher order mode. In our square array fibre, the fringevisibility at short wavelengths is lower than the hexagonal array butgenerally more consistent and higher over the whole wavelength range,only starting to perform below the FGIH-30-650s commercial fibre above680 nm.

We have presented two methods of fabricating endoscopic imaging fibresusing graded index preforms designed for telecommunications. Our fibrebased on a square array with several different core sizes positionedsuch that identical cores are not in close proximity gives significantlyimproved imaging performance over a broad wavelength range compared to awidely used commercial fibre with higher numerical aperture cores.Fluorescence images of USAF test target with a 3.1 μm line width arediscernible in our square array fibre in the wavelength band 520 nm to600 nm and 3.48 μm in the wavelength range 650 nm to 750 nm. Thisfibber's fabrication technique was based on a simplified stackingprocedure using rods derived from telecoms preforms of lower indexcontrast than commercial imaging fibres (NA of 0.3 compared to 0.4).This procedure therefore allows imaging fibres to be produced fromrelatively low cost starting materials, and potentially paves the wayfor cost-effective disposable imaging fibres for use in clinicalprocedures.

It may be understood that the present invention has been described abovepurely by way of example, and that modifications of detail can be madewithin the scope of the invention.

Each feature disclosed in the description and (where appropriate) theclaims and drawings may be provided independently or in any appropriatecombination.

The invention claimed is:
 1. A method of forming an imaging fiberapparatus comprising: arranging rods to form a plurality of stacks eachcomprising a respective plurality of rods, wherein: for each stack, therespective plurality of rods comprises rods having different core sizes,the rods of different core sizes being arranged in a selectedarrangement, and the rods of different core sizes being arranged suchthat each stack has a respective selected shape; wherein the selectedshape or shapes are such that the stacks stack together in a desiredarrangement; the method further comprising: drawing each of theplurality of stacks; stacking together the plurality of drawn stackstogether in the desired arrangement to form a further stack; drawing thefurther stack; and using the drawn further stack to form an imagingfiber apparatus, wherein the selected arrangement of the rods in eachstack and the selected shape or shapes of the stacks are such that thefurther stack comprises a repeating pattern of rods of different coresizes and for each of said rods of said further stack thenearest-neighbor rods for said rod have different core sizes to saidrod, wherein each stack comprises a plurality of rows and a plurality ofcolumns, and wherein, for each stack, each row of the stack comprises atleast one rod of each of a plurality of different core sizes, and eachcolumn of the stack comprises at least one rod of each of the pluralityof different core sizes.
 2. The method according to claim 1, wherein foreach stack, the respective plurality of rods comprises rods havingdifferent outer sizes.
 3. The method according to claim 1, wherein foreach stack, each of the respective plurality of rods has substantiallythe same ratio of core size to outer size.
 4. The method according toclaim 1, the method further comprising obtaining the rods by drawing atleast one preform.
 5. The method according to claim 4, wherein theobtaining of the rods comprises drawing a selected type of preform,wherein different core sizes are obtained by drawing the same selectedtype of preform differently.
 6. The method according to claim 1, whereinthe using of the drawn further stack to form an imaging fibre apparatuscomprises jacketing the drawn further stack.
 7. The method according toclaim 1, wherein the selected arrangement is such that, for each rod,the next-to-nearest neighbour rods for said rod have different coresizes to said rod.
 8. The method according to claim 1, wherein the oreach selected shape comprises a regular shape.
 9. The method accordingto claim 8, wherein the or each regular shape comprises at least one ofa square, a rectangle, a rhombus, a parallelogram, a hexagon, a regularpolygon.
 10. The method according to claim 1, wherein each of the stackshas substantially the same shape.
 11. The method according to claim 1,wherein each of the stacks has substantially the same selectedarrangement of rods.
 12. The method according to claim 1, wherein: theplurality of different core sizes comprises N different core sizes; eachstack comprises an N by N array of rods; and, for each stack, each rowof the stack comprises one rod of each of the N different core sizes;and each column of the stack comprises one rod of each of the Ndifferent core sizes.
 13. The method according to claim 1, wherein: theplurality of different core sizes comprises N different core sizes; eachstack comprises an array of yN columns by zN rows; and for each stack,each row of the stack comprises y rods of each of the N different coresizes; and each column of the stack comprises z rods of each of the Ndifferent core sizes.
 14. The method according to claim 1, wherein theforming of the optical fiber apparatus comprises performing at least onefurther stacking and drawing of the drawn further stack.
 15. The methodaccording to claim 1, wherein for each stack, arranging the respectiveplurality of rods to form the stack comprises positioning spacerelements between at least some of the respective plurality of rods. 16.The method according to claim 1, wherein each rod comprises at least oneof silica, Ge-doped silica, Fluorine doped silica, boron doped silica,Aluminum doped silica, silicate glass.
 17. The method according to claim1, wherein the rods have outer sizes between 0.5 mm and 10 mm,optionally between 1 mm and 5 mm.
 18. The method according to claim 1,wherein a width of each stack is between 10 mm and 100 mm.
 19. Themethod according to claim 1, wherein a numerical aperture of each rod isless than 0.35, optionally less than 0.32, further optionally less than0.3.